There are numerous applications with the need for extremely miniaturized motors that are able to make controlled fine positioning. For example, in consumer products extremely small, low weight, low power consumption and inexpensive motors are typically requested. The motion range is often in the order of millimeters and with an accuracy of micrometers.
Many of the micromotors are based on the actions of electromechanically active materials. A geometrical volume or shape change is achieved by applying some kind of electrical signals. Most common of such materials are piezoelectric materials.
There are numerous of approaches of electromechanical motors concerning the geometrical configuration and drive principles. One group of electromechanical motors utilizes resonant properties of the actuators and/or the body to be moved. Other motors use different kinds of quasi static driving principles, where at least some of the actuators are in static mechanical contact with the body to be moved during at least a part of a driving cycle. In general, resonant motors tend to be more power efficient, while the quasi static motors tend to have a higher positioning precision.
One particular geometric configuration that has proven to be useful in many applications comprises at least two actuating portions arranged essentially perpendicular to an interaction surface of the body to be moved. The actuating portions are arranged to be possible to move in at least a two-dimensional plane, perpendicular to the interaction surface of the body to be moved. The actuating portions are moved in an alternating fashion, creating a walking or stepping motion. One typical example of such a motor is presented in the published US patent application 2005/0179343 or references therein. Other example can be found e.g. in the German patent DE 44 08 618 or the U.S. Pat. No. 6,066,911.
The German patent DE 44 08 618 excites the actuating portions by electrodes evaporated on the sides of the piezoelectric material of the actuating portions. When applying a voltage across the piezoelectric material, the thickness of the piezoelectric material changes. As a consequence thereof, since the volume of the piezoelectric material essentially is constant, the piezoelectric material also changes the length of the actuating portion. By being able to activate two different parts of the actuating portion, crating a bimorph structure, the required motion can be achieved. The useful dimension change is created perpendicular to the applied voltage, denoted as a d31 excitation. Traditionally, this excitation has been considered as less power efficient, since a smaller dimension change, roughly 50%, is achieved in this direction compared to the direction of the applied electric field.
d31 excitation has also been used, e.g. in “Characteristics of a piezoelectric miniature motor” by M. Bexell et al., in Sensors an Actuators 75 (1999) pp. 118-130. d31 Actuator elements were mounted on a silicon wafer in a circle configuration for rotating a rotor. The actuator elements were contacted to conductors provided in the silicon wafer. The configuration was successfully operated, however, such construction relying on precision mounting is not very suitable for large volume production.
In the U.S. Pat. No. 6,066,911, a d33 excitation has instead being used. There, the dimension change parallel to the applied field has been utilized by placing electrodes of the piezoelectric material parallel to the interaction surface. By providing electrodes covering different parts of the actuating portion cross section, the required two-dimensional motion has been obtained. Furthermore, by providing electrodes inside the piezoelectric volumes, lower excitation voltages can be used to achieve a certain dimension change. A similar basic motion idea is also used in the published US patent application 2005/0179343.
When providing electrodes inside the actuating portion, considerations regarding the electrode edges have to be made. If the electrodes are permitted to extend all the way out to the side surface of the actuating portion, mechanical strength problems typically occur. The actuating portions tend to crack at the interface between the piezoceramic and the electrode. Furthermore, the actuator portions typically have to be provided with some electrically insulating and chemically protecting layer, in order to avoid electrical problems and corrosion of the electrodes. These problems are typically solved by providing the actuating portions with electrode-free volumes closest to the surface, i.e. letting the electrodes end at some distance to the surface of the actuating portion.